Slice a little piece off the top and bottom of the oranges so you have a stable cutting surface. Trim away all skin and pith with a small sharp knife without taking off too much of the fruit. Slice oranges horizontally and arrange on a serving platter.
Combine olive oil, white wine vinegar, and orange juice in a bowl. Season dressing with rosemary, salt, and pepper. Add cubed fennel and toss in the dressing. Arrange fennel on the orange slices. Layer onion rings on top of the fennel and sprinkle with dressing. Season with pepper and garnish with fennel fronds.